In machine construction using rivets, bolts, fasteners and other means of attachment, it is often beneficial to apply a sealant to the exposed portion of the fasteners to protect it from corrosion and to provide electrical insulation. The sealant can also function to seal around the fasteners.
In the manufacture of production aircraft, it is often required that the rivets, bolts, fasteners and other protrusions be covered with a curable sealant to prevent corrosion and to provide electrical insulation. Conventionally, elastomeric sealant has been applied by directing the uncured sealant directly onto the individual fasteners such as by use of brush or stick applicator and permitting the sealant to cure. This has been found to provide inconsistent results in that the uncured sealant, normally a polyurethane resin, is not uniformly applied and tends to sag or deform from a symmetrical configuration prior to curing. This inconsistency in the resulting sealant coatings accomplished by the prior art methods has resulted in thin and nonuniform regions over the outer surfaces of the fasteners, where specific minimum thicknesses for the sealant are required, it has been difficult to consistently maintain such standards using the prior art methods.
Attempts have also been made to apply a layer of sealant to fasteners by using a sealant-retaining shell. In this arrangement, uncured sealant is loaded into a cup-shaped molding shell and is then applied over the exposed portion of the fastener or other element and brought into contact with the panel or other surface through which the fastener is attached. The uncured sealant is entrapped between the exposed portion of the fastener and the shell thereby covering the fastener with sealant. This method is disclosed in U.S. Pat. No. 4,519,974 filed Aug. 12, 1983, and assigned to the assignee of the present application. Other attempts to use a molding shell to apply an uncured sealant to the exposed portion of a fastener are found in U.S. Pat. No. 3,470,787, to William L. Mackie and U.S. Pat. No. 2,710,113, to D. J. Pritchard. While the prior art system disclosed in the above referenced U.S. Pat. No. 4,519,974 has met with success, the system requires metering an amount of sealant into the molding shell substantially equal to the difference in the volume of the fastener head and the volume of the cavity defined by the shell. This is required such that there is no excess sealant which would otherwise extrude out of the shell. Thus, this arrangement, while avoiding the extrusion of excess sealant, as exists in the methods of Mackie and Pritchard, does require more accurate metering of sealant into the molding shell. This presents some problem where differing fastener heads are being coated.
Thus, a need exists for a method and apparatus for coating exposed fastener heads where the coating is uniform and provides a smooth contour over the fastener without presenting the problems attendant to systems requiring accurate metering or where excess sealant must be provided to assure complete coverage of the fastener being sealed.